A mass spectrometry (MS) system in general includes an ionization apparatus (or ion source) for ionizing components of a sample of interest, a mass analyzer for separating the ions based on their differing mass-to-charge ratios (or m/z ratios, or more simply “masses”), an ion detector for counting the separated ions, and electronics for processing output signals from the ion detector as needed to produce a user-interpretable mass spectrum. Typically, the mass spectrum is a series of peaks indicative of the relative abundances of detected ions as a function of their m/z ratios. The mass spectrum may be utilized to determine the molecular structures of components of the sample, thereby enabling the sample to be qualitatively and quantitatively characterized. In certain “hyphenated” or “hybrid” systems, the sample supplied to the ionization apparatus may first be subjected to a form of analytical separation. For example, in a liquid chromatography-mass spectrometry (LC-MS) system or a gas chromatography-mass spectrometry (GC-MS) system, the output of the LC or GC column may be transferred into the ion source through appropriate interface hardware.
The type of ionization apparatus deployed in the system depends on many factors. Examples of ionization techniques implemented by different types of ionization apparatuses include photo-ionization (PI), electrospray ionization (ESI), chemical ionization (CI), field ionization (FI), electron ionization (EI), laser desorption ionization (LDI), and matrix-assisted laser desorption ionization (MALDI). Some of these techniques are effective at or near atmospheric pressure and others are effective at vacuum pressure, while some may be adapted for implementation in either regime.
Ultraviolet (UV) PI is becoming recognized for its ability to ionize many chemical species, both polar and non-polar, with reduced ion suppression and retention of high sensitivity and dynamic range, as compared for example to widely used ESI. With the appropriate choice of photon wavelength (energy), efficient analyte ionization and low levels of undesired ionization of common LC solvents can be achieved simultaneously. Common UV PI sources, however, use a low internal-pressure gas discharge lamp, e.g. krypton (10.2 eV), in an atmospheric pressure ionization chamber. These sources are limited in their use mainly by low-intensity radiation (photon flux), ambient optical absorption of the UV flux, and unwanted ion chemistry in the high-pressure environment.
Therefore, there is a need for PI sources capable of producing higher photon flux levels and ionization efficiency with minimal ionization of non-analytical molecules, and which are effective for providing the advantages of UV PI in both low-pressure and high-pressure environments.